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In testimony to the House Intelligence Committee on Thursday, Hill said based on their questions and statements some members of the panel appear to believe that Russia and its security services did not conduct a campaign against the United States during the 2016 presidential race, and that perhaps Ukraine did.
Some Republican members of the Democratic-led committee have advanced the discredited theory, which has been embraced by Trump and his political allies.
"This is a fictional narrative that has been perpetrated and propagated by the Russian security services themselves," said Hill, who until July served as the director for European and Russian affairs at the White House National Security Council.
"In the course of this investigation, I would ask that you please not promote politically driven falsehoods that so clearly advance Russian interests," she said in the last scheduled public hearing in the House inquiry.
The notion that Ukraine and not Russia interfered in 2016 was one of two issues that the US president urged Ukrainian President Volodymyr Zelenskiy to investigate in a July 25 phone call that is at the heart of the impeachment effort.
Trump also pressed Ukraine on the call to investigate Joe Biden, a leading contender for the Democratic nomination to face Trump in the 2020 presidential election, and his son Hunter, who had served on the board of Ukrainian gas company Burisma.
The inquiry is also examining whether Trump's withholding of $US391 million ($NZ610 million) in security aid to Ukraine, approved by Congress to fight Russia-backed separatists in the eastern part of the country, was meant to pressure Zelenskiy to conduct the probes.
Democrats say Trump's dealings with Ukraine amount to an abuse of power to pressure a vulnerable US ally to dig up dirt on a domestic political rival.
Trump has denied wrongdoing, publicly criticised witnesses and described the impeachment proceedings as a "witch hunt." His fellow Republicans call the proceedings a "sham."
'HAND GRENADE' GIULIANI
David Holmes, a staffer from the US Embassy in Ukraine, testified on Thursday that his work started to become overshadowed in March by Trump's personal lawyer, Rudy Giuliani, who was pushing Ukraine to carry out the two probes.
In her testimony, Hill recalled a conversation she had with Trump's former National Security Adviser John Bolton about Giuliani's activities.
She said she had already expressed regrets to Bolton about Giuliani’s "smear campaign" against Marie Yovanovitch, the former US ambassador to Ukraine, who Trump withdrew prematurely from Kiev in May.
Bolton, she said, looked pained and "basically indicated with body language that there was nothing much that we could do about it. And then, in the course of our discussion said Rudy Giuliani was a hand grenade that was going to blow everyone up."
Asked to elaborate on what she thought Bolton meant, Hill replied that what Giuliani was say was “pretty explosive” and that the former New York mayor “was clearly pushing forward issues and ideas that would, you know, probably come back to haunt us and in fact I think that that's where we are today.”
US intelligence agencies and former Special Counsel Robert Mueller have determined that Russia interfered in 2016 with a campaign of hacking and propaganda intended to sow discord in the United States, boost Trump's candidacy and harm his Democratic opponent Hillary Clinton.
Mueller's team brought criminal charges against 12 Russian intelligence officers in the hacking effort, accusing them of covertly monitoring employee computers and planting malicious code, as well as stealing emails and other documents.
Hill warned lawmakers that Russia is gearing up to repeat its election interference activities in 2020.
"We are running out of time to stop them," she said.
Like a number of career government officials who have already testified, Hill said she prides herself as a nonpartisan foreign policy expert who has served Republican and Democratic presidents.
The committee's top Republican, Devin Nunes, nonetheless took issue with her comments, saying a report by Intelligence Committee Republicans "analysed the 2016 Russia meddling campaign."
"Needless to say, it's entirely possible for two separate nations to engage in election meddling at the same time and Republicans believe we should take meddling seriously by all foreign countries, regardless of which campaign is the target," Nunes said.
Embassy staffer Holmes also testified about a July 26 phone call in which he said he overheard Trump ask Gordon Sondland, US ambassador to the European Union, about the status of the investigations. He said he heard Trump's voice on the call at a Kiev restaurant.
"So, he's gonna do the investigation?" Trump asked Sondland, referring to Zelenskiy, Holmes said.
"He's gonna do it," replied Sondland, he said.
Sondland added that the Ukrainian president would do "anything you ask him to," Holmes said.
Holmes said Sondland told him after the call that Trump was interested in "big stuff" in Ukraine, defining that as "like the Biden investigation that Giuliani is pushing."
On Wednesday, Sondland testified he could not remember the precise details of the call Holmes overheard, but took issue with his recollection that he had talked about the Bidens.
"I do not recall mentioning the Bidens. That did not enter my mind. It was Burisma and 2016 elections," Sondland said. Trump says he does not remember the call with Sondland.
With no more witnesses scheduled to appear, Thursday's hearing could mark the end of the investigation phase of the impeachment proceedings.
Once the committees running the inquiry agree the probe is complete, the evidence then goes to the House Judiciary Committee which will consider whether to draft formal charges, known as articles of impeachment - against Trump. Democratic lawmakers say that is unlikely to happen before Thanksgiving on November 28.
If the full House then votes to impeach Trump, the charges will go to the Republican-controlled Senate for a trial on whether to remove him from office. Few Republican senators have broken with the president, however.